1. Field of the Invention
The present invention relates an image reading device for use in a digital copier, a facsimile apparatus, a scanner or the like. More particularly, the present invention relates to an image reading device of the type reading an image with a one-dimensional line sensor by optically scanning a document surface and capable of correcting irregularity and other shading characteristics of read data with high accuracy and a shading correcting method for the same.
2. Description of the Background Art
Generally, when an image reading device installed in, e.g., a digital copier, a facsimile apparatus or a scanner reads a document, its output level is irregular even if the document is of the same color or the same density. This irregularity is a scribable to various causes including a difference in sensitivity between a plurality of pixels, which constitute an image sensor, aberration of a lens system and irregular illumination of a light source that illuminates a document. It is therefore a common practice with the image reading device to execute shading correction for electrically correcting a difference in output level between pixels.
A shading correcting method customary with the image reading device will be described hereinafter. A reference white plate, positioned in the vicinity of a document reading position, is illuminated before the actual document image so as to read the resulting reflection and store it as pixel-by-pixel shading correction data. To make such shading correction accurate, it is necessary that the shading correction data-derived from the reference white plate contains a minimum of noise with respect to the true level. For example, if the noise of the shading correction data is great, then image data read, but corrected by the shading correction data, becomes output image data effected by the noise of the shading correction data.
In light of the above, it is a common practice to minimize the influence of noise by generating shading correction data on the basis of a plurality of lines of data derived from the reference white plate. For example, shading correction data may be generated by simple averaging or weighted averaging executed with the plurality of lines of data, as taught in, e.g., Japanese Patent Laid-Open Publication Nos. 05-110848 and 11-289432. However, such a conventional shading correcting method has some problems left unsolved, as will be described specifically later.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 2002-300393, 2002-300392 and 11-103390.